The backlight technology of LCD is developed continuously as the display technique is improved constantly. A backlight source of a conventional liquid crystal display adopts a cold cathode fluorescent lamp (CCFL). But the CCFL backlight source has disadvantages such as relatively poor color rendition ability, low luminous efficiency, high discharge voltage, poor discharge property at low temperature, long time for heating till stable gray scale and so on. The backlight source technology using a light emitting diode (LED) backlight source has been developed.
FIG. 1 is a conventional LED backlight driving circuit applied in a liquid crystal display. As shown in FIG. 1, the LED backlight driving circuit includes a boost circuit, a LED controller, a capacitor C1′ and a LED strip. The boost circuit includes an inductance L′, a diode D′, a first transistor Q1′ and a first resistor R1′. One end of the inductance L′ receives a direct-current voltage Vin input from a power source, the other end of the inductance L′ is connected to an anode of the diode D′ and a drain electrode of the first transistor Q1′. A gate electrode (control end) of the first transistor Q1′ is driven by a first control signal provided by the LED controller. The source electrode of the first transistor Q1′ and the ground are electrically connected by the first resistor R1′. A cathode of the diode D′ is electrically connected to a positive terminal of the LED strip. The cathode of the diode D′ is further connected with the ground by the capacitor C′. A negative terminal of the LED strip is further connected with a second transistor Q2′. A drain electrode of the second transistor Q2′ is connected to the negative terminal of the LED strip. A source electrode of the second transistor Q2′ and the ground are electrically connected by a second resistor R2′. A gate electrode of the second transistor Q2′ is driven by a second control signal provided by the LED controller. The operational current of the LED strip can be increased or decreased by adjusting a duty ratio of the second control signal to control brightness of the LED strip.
The inventor of the disclosure discovers the number of required LED lamps is increasing with enlargement of panels, demands of outdoor display or commercial display in the process of the LED backlight driving circuit above, such as the number of LED lamps included in the LED strip exceeds 16. The LED lamps are series connection, which lead to increase of an output voltage Vout of the inductance L′ to drive the LED strip after boosting the voltage, such as over 90V or 100V, so that the first transistor Q1′ and the diode D′ experience a high level of stress, resulting in shortening life of the first transistor Q1′ and the diode D′, or even causing damage of the LED backlight driving circuit. Moreover, conversion efficiency of the boost circuit is inversely proportional to boost voltage, which means the conversion efficiency is lower while the voltage is higher. The conversion efficiency of the boost circuit will be reduced, energy is wasted.